"But now these sects are our whetstones and polishers; they whet and grind our faith and doctrine so that, smooth and clean, they sparkle as a mirror. Moreover we also learn to know the devil and his thoughts and become prepared to fight against him."
What Luther Says, An Anthology, 3 vols., ed., Ewald Plass, St. Louis: Concordia Publishing House, 1959, III, p. 1269.